Measuring the temperature of tissues, veins or other body parts at remote locations during surgical, electrophysiological and other invasive and minimally invasive procedures can provide critical information to the physician or surgeon. One such procedure is the detection and location of inflamed plaque in an artery.
An atherosclerotic plaque is a thickened area in the wall of an artery. Typically, patients who have died of coronary disease may exhibit as many as several dozen atherosclerotic plaques. However, in most instances of coronary disease, it is found that only one of the atherosclerotic plaques has ruptured, fissured, or ulcerated. The rupture, fissure, or ulcer causes a large blood clot to form on the inside of the artery, which may completely occlude the flow of blood through the artery, thereby injuring the heart or brain. A major prognostic and diagnostic dilemma for the cardiologist is how to predict which plaque is about to rupture.
The rupture process is not completely understood, but it is known that the plaques most likely to rupture are those with inflamed surfaces or a high density of activated macrophages and a thin overlying cap. Van der Wal, et al., Circulation 89:36-44 (1994); Shah, et al., Circulation 244 (1995); Davies, et al., Br Heart J 53:363-373 (1985); Farb, et al., Circulation 92:1701-1709 (1995); Van Damme, et al., Cardiovasc Pathol 3:9-17 (1994). Plaques with inflamed surfaces are thought to be located at junctures where pools of cholesterol meet a more cellular and fibrous part of the plaque. Typically, inflammatory cells, which produce heat, have been found at these junctures. Since these inflammatory cells release enzymes capable of degrading the collagen and other components of the extracellular matrix, it is thought that they are crucial to the process of plaque rupture or fissuring.
Thus, plaques which are believed to be at risk of rupturing, or plaques with inflammatory cells, are known to have a higher temperature than the surrounding tissue by 1-2° C. Accordingly, the detection of such inflamed plaques is helpful in the prediction and subsequent prevention of plaque rupture. U.S. Pat. Nos. 5,935,075 and 5,871,449 disclose devices for locating inflamed plaque based upon a temperature measurement. Both patents disclose infrared devices and are not readily adaptable to existing catheter systems.
Another procedure where knowledge of temperature at the precise location of the therapy is ablation, which is the technique used to correct cardiac arrhythmia. Cardiac arrhythmia is an electrical malfunction of the heart. Electrophysiological therapy treats cardiac arrhythmia by ablating the tissue area that is the source of the electrical malfunction. To ablate the tissue, a physician performing the electrophysiological therapy steers an ablation catheter through a vein or an artery into the interior region of the heart. The catheter has an ablating element (e.g., an electrode or a fiber-optic bundle) along the distal end of the catheter which delivers energy (e.g., radio frequencies or laser light) to the targeted tissue. The delivered energy heats the tissue and forms a lesion.
Temperature is a critical parameter in achieving success with the ablation procedure. The lesion size and shape are a function of the temperature of the ablated tissue, the surrounding tissue, and/or the ablating element. Thus, temperature is monitored during the procedure. The catheter includes a temperature sensor to measure the temperature of the tissue near the ablating element during the procedure. It is known to use a thermistor or a thermocouple at the end of the catheter to measure the temperature.
The ablation catheter typically is connected to a controller/generator unit by a medical grade cable with an adapter specific to the controller/generator unit. The generator delivers the energy necessary for ablation and the controller receives temperature-related signals from the thermistor or the thermocouple. The controller may further monitor electrical activity of the heart during the ablation procedure. The controller and the generator can be a single unit, or the two functions can be performed by two separate units.
Radio frequency (RF) energy is one type of ablating energy used in lectrophysiological therapy. The RF ablation generator/controller units currently available are either thermistor-based or thermocouple-based. That is, known RF generator/controller units can display temperature based on signals provided by a temperature sensor in the catheter, but they can do so only for a particular type of temperature sensor, either a thermistor or a thermocouple. One example of a combination ablation/temperature sensor catheter is disclosed in U.S. Pat. No. 5,833,688, assigned to the assignee of the present application.
Other types of sensors include the use of fiber-optic catheters having a fluorometric sensor at a distal end of the catheter. One example is disclosed in U.S. Pat. No. 5,012,809. Such sensors operate by using a light source to excite fluorescent material disposed at a distal end of the catheter. The excited fluorescent material in turn emits light which is transmitted back through the fiber-optic cable. The lifetime of the fluorescence is used to calculate the temperature at the distal end of the catheter. However, the catheter disclosed in the '809 patent utilizes an incandescent light source, and photodetectors which require the use of amplifiers. The algorithms used to calculate temperature are dependent upon the polymer/fluophor matrix used in construction of the catheter.
U.S. Pat. Nos. 4,448,547 and 4,560,286 also disclose the use of fluorescent materials or phosphors to measure temperature. However, the apparatus as disclosed in these patents require the use of a radioactive material or a source of ultraviolet light to excite the phosphor or fluorescent material.
Accordingly, there is a need for an improved fluorescent temperature sensor which can be incorporated into currently available catheter designs in an inexpensive and efficient manner and further which can be operated with minimal trauma to the artery.